This project will investigate changes in responses of bone marrow-derived lymphocytes (B cells) with aging. Specifically, we will investigate changes in the B cells themselves and/or changes in the mouse lymphoid system which result in the appearance of B cells which can be activated by anti-immunoglobulin reagents. These cells are detectable in the normal mouse spleen of approximately 7 months of age and are rarely found in younger mice. Special emphasis will be given to the study of apparent changes in Fc receptors on the B cells and the association these changes have with aging and with anti-Ig-induced B cell activation.